Excimer lasers are pulsed, gas-discharge lasers, and operate on a gas mixture containing rare gasses, for example: some combination of helium, neon, argon, krypton and xenon gases; and a halogen gas, for example fluorine, hydrogen chloride, etc. Under the appropriate conditions of electrical stimulation and high pressure, a pseudo-molecule called an excimer, or in the case of noble gas halides an exciplex, is created, which can only exist in an energized state and may give rise to laser light in the ultraviolet (UV) range.
There are four most commonly used excimer wavelengths, which are dependent upon the active gases in the laser, i.e. Argon Fluoride (193 nm), Krypton Fluoride (248 nm), Xenon Chloride (308 nm), and Xenon Fluoride (351 nm). The invention could also be used with a fluorine laser (157 nm), although this is not actually an excimer.
In excimer lasers containing fluorine, the generation of hydrogen fluoride (HF) is a significant detriment to laser performance because HF is a strong absorber of UV light produced by the excimer laser, and because HF is much more chemically reactive than fluorine gas. The increased reactivity of HF accelerates wear on components within the laser vessel, and therefore may shorten the service life thereof.
HF forms when fluorine reacts with water and other hydrogen containing compounds. Typically small amounts of water vapor are introduced into the excimer laser when a fresh gas fill is added through contamination of the laser gases with water vapor, or when the laser vessel is opened for service. Simply evacuating the vessel may be effective in removing most laser gas contaminants, e.g. carbon tetrafluoride (CF4), which don't have a storage reservoir within the laser vessel, but HF is extraordinarily reactive and polar, and tends to stick to the interior surface of the laser vessel and any dust inside the laser. Accordingly, while most contaminants are diluted by the ratio of the operating pressure to the evacuation pressure each time a fresh fill of gas is added, HF is much less diluted.
For a typical operating pressure of 5 bar, and an evacuation pressure of 0.1 bar, contaminants are diluted by a factor of 50. If the contaminants, other than HF, have a concentration of 250 ppmv when the gas fill is replaced, then the fresh fill will start with only 5 ppmv of these contaminants. For HF, the dilution method does not work as well, because the HF is in equilibrium with the interior of the laser vessel at a pressure of a few millibar. For example, a residual pressure of 1 millibar is equivalent to 200 ppmv concentration in a 5 bar laser gas mix. Evacuating the laser vessel to a pressure below the equilibrium pressure of the HF is not practical for industrial use. When a fresh fill is added to the laser vessel, and the laser warms up, the absorbed HF partially desorbs and contaminates the fresh gas fill.
Existing methods for gas purification in an excimer laser include cryogenic trapping of contaminants, such as HF and CF4, and purification of gasses being introduced into the laser vessel. Moreover, care may be taken with material choices inside the laser vessel to minimize reactions with the halogen components of the gas mix.
U.S. Pat. No. 8,929,419, issued Jan. 6, 2015 in the name of Dean et al, which is incorporated herein by reference, describes removal of contaminants and fluorine to allow recycling of the noble gas component of the excimer laser gas mixture.
An object of the present invention is to overcome the shortcomings of the prior art by reducing the concentration of HF within an excimer laser.